The present invention relates to the reaction of carbon monoxide and water to produce hydrogen and carbon dioxide. In particular, it is directed to such process catalyzed by a rhodium or iridium component with an iodide promoter.
A number of heterogeneous catalysis systems are known for use in the water-gas shift reaction, employing oxides of such metals as nickel, iron and cobalt, at elevated temperatures such as 400.degree. C. The water-gas shift reaction is used to produce hydrogen, particularly for use in the synthesis of ammonia.
As described in application of Arnold Hershman, Ser. No. 801,711, filed of even date herewith, it was found that the water-gas shift reaction can be effectively catalyzed by iodide-promoted rhodium or iridium catalysts at relatively low temperatures and pressures, particularly in the absence of other materials readily capable of carbonylation. The temperatures employed, such as 125.degree. to 225.degree. C., favor the production of hydrogen.
The catalysts have good activity and give good reaction rates at relatively mild temperatures. The process has other advantages characteristic of a hemogeneous liquid phase catalytic reaction.
It is known that the water-gas shift reaction may occur to some extent if water is present during carbonylation of various substrates, e.g. methanol, in the presence of carbonylation catalysts, but this reaction is very minor when rhodium or iridium catalysts are used. However iodide-promoted catalysts are very effective for the water-gas shift reaction when methanol and similar readily carbonylatable substrates are absent and substantial amounts of water are present.
General carbonylation conditions are useful herein (i.e., conditions used in carbonylation reactions) with iodide-promoted rhodium or iridium catalysts, with regulation of particular parameters to obtain advantages and desired results as described herein. Rhodium and iridium carbonylation catalysts are taught in Paulik et al U.S. Pat. Nos. 3,772,380 and 3,769,329, and Craddock et al U.S. Pat. Nos. 3,816,488, 3,816,489, 3,579,551 and 3,579,552.
It has now been discovered that concentrations and conditions have a very pronounced effect upon the water-gas reaction rates obtainable with the iodide-promoted rhodium and iridium catalysts utilized herein. In particular, as will be further discussed hereinbelow, the water concentrations, and the relationship of the water concentrations to the iodide concentration, having a strong effect upon reaction rates. The effect of acidity, in terms of the Hammett acidity function, upon the reaction rate is also discussed and factors affecting the acidity are described and illustrated.
Substrates which are readily carbonylatabe are substantially absent during the present process. Thus methanol and other hydroxyl-containing compounds or olefinic hydrocarbons and other materials with olefinic bonds are excluded in the present process.
In accordance with the present invention, carbon monoxide and water are reacted at temperatures from about 50.degree. C. to 300.degree. C., preferably 125.degree. C. to 225.degree. C., and at partial pressures of carbon monoxide from 1 p.s.i.a. to 15,000 p.s.i.a., preferably 5 p.s.i.a. to 3,000 p.s.i.a., and more preferably 25 p.s.i.a. to 1,000 p.s.i.a., although higher pressures may be employed, in the presence of a catalyst system comprised of rhodium-or iridium-containing component, and a promoter portion i.e., in iodide. A temperature range of 150.degree. to 200.degree. C. is particularly suitable, and at pressures say of 100 p.s.i.a. to 1,000 p.s.i.a. The iodide may be derived from iodine or iodine compounds.
For purposes of the present invention, the catalyst system essentially includes a rhodium or iridium component and a halogen component in which the halogen is iodine. Generally, the rhodium component of the catalyst system of the present invention is believed to be present in the form of a coordination compound of rhodium with an iodide component providing at least one of the ligands of such coordination compound. In addition to the rhodium or iridium and iodide, in the process of the present invention, these coordination compounds also generally include carbon monoxide ligands thereby forming such compounds or complexes, for example, as [Rh(CO).sub.2 I].sub.2 and the like. Other moieties may be present if desired. Generally, it is preferred that the catalyst system contain as a promoting component, an excess of iodide over that present as ligands in the coordination compound. The terms "coordination compound" and "coordination complex" used throughout this specification mean a compound or complex formed by combination of one or more electronically rich molecules or atoms capable of independent existence with one or more electronically poor molecules or atoms, each of which may also be capable of independent existence.
The essential rhodium or iridium and iodide component of the catalyst system of the present invention may be provided by introducing into the reaction zone a coordination compound of rhodium or iridium containing iodide ligands or may be provided by introducing into the reaction zone separately a rhodium or iridium compound and an iodine compound. Among the materials which may be charged to the reaction zone to provide the rhodium component of the catalyst system of the present invention are rhodium metal, rhodium salts and oxides, organo rhodium compounds, coordination compounds of rhodium, and the like. Specific examples of materials capable of providing the rhodium constituent of the catalyst system of the present invention may be taken from the following non-limiting partial list of suitable materials.
RhCl.sub.3 PA0 RhBr.sub.3 PA0 RhI.sub.3 PA0 RhCl.sub.3 .multidot.3H.sub.2 O PA0 RhBr.sub.3 .multidot.3H.sub.2 O PA0 Rh.sub.2 (CO).sub.4 Cl.sub.2 PA0 Rh.sub.2 (CO).sub.4 Br.sub.2 PA0 Rh.sub.2 (CO).sub.4 I.sub.2 PA0 Rh.sub.2 (CO).sub.8 PA0 Rh[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CO)I PA0 Rh[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CO)Cl PA0 Rh metal PA0 Rh(NO.sub.3).sub.3 PA0 RhCl[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CH.sub.3)].sub.2 PA0 Rh(SnCl.sub.3)[(C.sub.6 H.sub.5).sub.3 P].sub.3 PA0 RhCl(CO)[(C.sub.6 H.sub.5).sub.3 As].sub.3 PA0 Rhi(co)[(c.sub.6 h.sub.5).sub.3 sb].sub.2 PA0 [(n-C.sub.4 H.sub.9).sub.4 N][Rh(CO).sub.2 X.sub.2 ] where X--Cl,Br.sup.-,I-- PA0 [n-C.sub.4 H.sub.9).sub.4 As][Rh.sub.2 (CO).sub.2 Y.sub.4 ] where Y--Br.sup.-,I-- PA0 [(n-C.sub.4 H.sub.9).sub.4 P][Rh(CO)I.sub.4 ] PA0 Rh[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CO)Br PA0 Rh[n-C.sub.4 H.sub.9).sub.3 P].sub.2 (CO)Br PA0 Rh[(n-C.sub.4 H.sub.9).sub.3 P](CO)I PA0 RhBr[(C.sub.6 H.sub.5).sub.3 P].sub.3 PA0 RhI[(C.sub.6 H.sub.5).sub.3 P].sub.3 PA0 RhCl[(C.sub.6 H.sub.5).sub.3 P].sub.3 PA0 RhCl[C.sub.6 H.sub.5).sub.3 P].sub.3 H.sub.2 PA0 [(c.sub.6 h.sub.5)p].sub.3 rh(CO)H PA0 Rh.sub.2 O.sub.3 PA0 [rh(C.sub.2 H.sub.4).sub.2 Cl].sub.2 PA0 K.sub.4 rh.sub.2 Cl.sub.2 (SnCl.sub.3).sub.4 PA0 K.sub.4 rh-Br.sub.2 (SnBr.sub.3).sub.4 PA0 K.sub.4 rh.sub.2 I.sub.2 (SnI.sub.3).sub.4
With those materials listed above as capable of providing the rhodium component which do not contain an iodine component, it will be necessary to introduce in the reaction zone such iodide component. For example, if the rhodium component introduced is rhodium metal or Rh.sub.2 O.sub.3, it will be necessary to also introduce a halide component such as methyl iodide, hydrogen iodide, iodine or the like.
As noted above, while the halogen component of the catalyst system may be in combined form with the rhodium, as for instance, as one or more ligands in a coordination compound of rhodium, it generally is preferred to have an excess of halogen present in the catalyst system as a promoting component. By excess is meant an amount of halogen greater than 2 atoms of halogen per atom of rhodium in the catalyst system. This promoting component of the catalyst system consists of iodine and/or iodine compounds, such a hydrogen iodide, alkyl- or aryl iodide, metal iodide, ammonium iodide, phosphonium iodide, arsonium iodide, stibonium iodide and the like. The iodide of the promoting component may be the same or different from that already present as ligands in the coordination compound of rhodium.
Iodine or iodide compounds are suitable for the promoter portion of the catalyst, but those containing iodide are preferred, with hydrogen iodide constituting a more preferred member. Accordingly, suitable compounds providing the promoter portion of the catalyst system of this invention may be selected from the following list of preferred iodide and/or iodine containing compounds:
RI.sub.n where R=any alkyl, alkylene or aryl-group, e.g., Ch.sub.3 I, C.sub.6 H.sub.5 I, Ch.sub.3 Ch.sub.2 I, ICH.sub.2 I, etc. (n is 1-3) ##STR1## Where R=any alkyl or aryl-group, e.g., ##STR2## R.sub.4 MI, R.sub.4 MI.sub.3, or R.sub.3 MI.sub.2 where R=hydrogen or any alkyl- or aryl-group, M=N, P, As or Sb, eg. NH.sub.4 I, PH.sub.4 I.sub.3, PH.sub.3 I.sub.2 (C.sub.6 H.sub.5).sub.3 PI.sub.2, and/or combinations of R M and I.
It is recognized that some moieties of suitable promoter compounds may be subject to carbonylation, but in view of the relatively small amounts generally involved, such carbonylation will not induly interfere with the reaction of carbon monoxide with water which may proceed after more readily reacting materials have been consumed.
Similarly, iridium components of catalyst systems can be charged as iridium metal, iridium salts and oxides, organo iridium compounds, coordination compounds of iridium, and the like, specific examples being IrCl.sub.3, IrBr.sub.3, IrI.sub.3, IrCl.sub.3 .multidot.H.sub.2 O, and the various other materials illustrated by substituting Ir for Rh in any of the materials in the nonlimiting list of rhodium materials disclosed herein.
Generally it is preferred that the process of the present invention be carried out in an acidic reaction medium, as such appears characcteristic of, or to facilitate, conversions involved in the catalysis. The acidity is generally such as to be capable of forming alkyl halide from alcohol or olefin if such were added to the reaction medium, and particular acidity factors are further described herein.
The catalyst can be formed in situ in the reactor, or be formed separately. For instance, a catalyst precursor, e.g., RhCl.sub.3 .multidot.3H.sub.2 O or Rh.sub.2 O.sub.3 .multidot.5H.sub.2 O, may be dissolved in a dilute aqueous acid solution, e.g., HCl, acetic acid, etc., as solvent. Then the solution of the rhodium compound is heated, for example, to 60.degree. C.-80.degree. C., or in general at a temperature below the boiling point of the solvent, with stirring. A reducing agent such as carbon monoxide is bubbled through the said solution and subsequently, the iodine promoter is added as described herein.
Another embodiment of the present invention employs compounds of monovalent rhodium or iridium initially, wherein the transformation to active catalyst does not involve a change of valence. For example, monovalent rhodium salts such as Rh[(C.sub.6 H.sub.5).sub.3 P].sub.3 Cl, [Rh(C.sub.6 H.sub.5).sub.3 P].sub.3 (CO)Cl, Rh(C.sub.6 H.sub.5).sub.3 P].sub.3 H and [Rh(CO).sub.2 Cl].sub.2 are dissolved in a suitable solvent and carbon monoxide is subsequently passed through a solution that is preferably warmed and stirred. Subsequent addition of a solution of the halogen promoter, e.g., alkyl iodide, elemental iodine, aqueous HI, etc., results in formaion of an active carbonylation catalyst solution containing the necessary rhodium and iodide components. Iridium can be substituted for rhodium in any of the illustrative rhodium compounds herein above, and the preparation carried out as described.
In carrying out the reaction in liquid phase, any solvent compatible with the catalyst system and not interfering with the reaction may be employed. The preferred solvent and liquid reaction medium is a mono-carboxylic acid having 2-20 carbon atoms, e.g. acetic, propionic, nonanoic, naphthoic, and elaidic acids, including isomeric forms. Other inert solvents can be employed, although in general it will be preferred to select solvents to provide a homogeneous medium with the water present, either with individual solvents, or by use of co-solvents, if appropriate.
The present process results in the production of hydrogen, along with carbon dioxide, and the gaseous hydrogen can readily be recovered as off-gases, from batch, continuous or semi-continuous procedures. The hydrogen can be separated from the carbon monoxide, carbon dioxide or other components of the gas stream, or also for some applications be used as a reactant stream without purification. The water-gas shift reaction itself can be run to reasonably high conversions, or in stages, to consume the carbon monoxide and give a product with fairly low concentrations of this component. However, it is preferred to operate with substantial carbon monoxide pressures, which mitigates against high conversions. The carbon dioxide product is also suitable for some uses, but may for some applications simply be removed from the product gas by scrubbing, absorption or similar procedures.
The reaction rate is dependent upon catalyst concentration and temperature. Concentrations of the rhodium or iridium compound or the first component of the catalyst system in the liquid phase between 10.sup.-6 moles/liter and 1 mole/liter, are normally employed, with the preferred range being 0.001 mole/liter to 0.5 mole/liter. Higher concentrations than those set forth herein may, however, be used if desired. Higher temperatures also favor higher reaction rates.
The concentration of the second or promoter component of the catalyst system can vary widely but will generally be selected to give good rates with the particular catalyst concentration and other conditions, and usually being within the range of about 10.sup.-2 moles/liter to 10 moles/liter based on iodine atom, or more narrowly 0.1 mole/liter to 5 mole/liter. The preferred amounts will be affected by the particular promoter, being influenced by its reactivity and availability of the iodine content, as well as by its effect on acidity or other characteristics of the reaction medium, but are often about 0.1 mole/liter to 2 moles/liter. The relationship of promoter and its acidity, and the water concentrations, and their significance is further described hereinbelow.
For the water-gas shift reaction, one mole of carbon monoxide is required for each mole of water, but either component can be used in excess. The reaction can suitably be conducted with ratios of the reactants suitable for good reaction rates, with additional reactants supplied to the reaction to replace those consumed. In practice appropriate pressures of carbon monoxide may simply be selected to give good rates. However, for economic reasons or to have product suitable for particular application, it may be appropriate to select conditions to give high conversions of the carbon monoxide, which may include use of excess water. On the other hand, CO pressure contributes to catalyst stability and may affect reaction rate. The water will generally be employed in concentrations suitabe to give desired reaction rates. Generally, a substantial amount of water will be present ranging up to dilute aqueous solutions containing the catalytic components, but adjusted along with other factors as taught herein to give desirable reaction rates.
Procedures employed in the following examples which illustrate the invention were carried out in an autoclave reactor with a 1500 ml. capacity, with a liquid charge of 500 ml., and an agitator speed of 500 rpm. In general the procedures were carried out in the same manner for comparison purposes. A catalyst slurry was formed by mixing hydrated rhodium oxide with concentrated hydriodic acid (57%), and stirring for 10 minutes under a nitrogen purge, followed by water and acetic acid with stirring under the nitrogen purge for an additional 10 minutes. The catalyst slurry was then transferred to the autoclave reactor with agitation and additional water, acetic acid, HI or other components as necessary to attain the concentrations as reported herein.
The reactor was flushed with CO by pressuring and venting, and charged to desired partial pressure of CO, and stirred and heated to desired temperature, with a small flow of off-gas. When the desired temperature was attained, the off-gas rate, measured by a rotameter, was increased to about 1000 ml./minute. The off-gas was sampled by mass spectrometer and a chromatographic analyzer. Rate determinations were made when the off-gas showed a nearly constant composition, being calculated on the CO.sub.2 content as analyzed by the chromatographic analyzer. The mass spectrometric analyses for CO.sub.2 was generally in good agreement with the chromatographic analysis and in most cases the mass spectrometric analyses showed the hydrogen and CO.sub.2 content to be substantially equivalent. The water concentrations were determined by anaylsis after the reaction was stopped in some cases, as well as by measurement of the initial charge. The procedures were generally completed in about one hour.